The 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) is a project for improving the UMTS (Universal Mobile Telecommunication System)-standard, in order to cope with future requirements in terms of improved services, such as higher data rates, improved efficiency, and lower costs. The radio access network of an LTE system is commonly referred to as an E-UTRAN (Evolved Universal Terrestrial Radio Access Network), and the UTRAN is the radio access network according to the UMTS.
In a UTRAN, a user equipment, UE, is connected to a radio base station commonly referred to as a Node B, and the Node B is connected to a network node, typically an RNC (Radio Network Controller). The RNC is connected to a core network, which comprises both a circuit switched service domain and a packet switched service domain.
In a radio access network according to the 2nd generation, e.g. a GERAN (Global System for Mobile communication, GSM, Edge Radio Access Network), the radio base station is referred to as a BTS (Base Transceiver Station), which is connected to a core network via a network node, typically a Base Station Controller (BSC), the core network also comprising both a circuit switched service domain and a packet switched service domain.
However, in the above-mentioned E-UTRAN, a UE is connected to a radio base station commonly referred to as an eNodeB (evolved NodeB), which is connected to the core network node, and the core network comprises only a packet switched service domain.
Thus, the E-UTRAN in the LTE uses a RAT (Radio Access Technology) that is not CS (Circuit Switched) capable. Instead, CS-based services, e.g. conventional circuit-switched voice calls, is replaced e.g. by VoIP in and LTE system.
For this reason, an LTE-capable UE that is attached to an E-UTRAN and wishes to initiate a CS service, e.g. a conventional voice call, has to transfer to a radio access network, RAN, with a CS-capable radio access technology, RAT, e.g. a UTRAN or a GERAN. Thus, the serving eNodeB will transfer the UE to a CS-capable radio access network in which the UE has radio coverage, and this RAN will provide the requested CS-service to the UE.
FIG. 1 schematically illustrates an UE 11 having an overlapping UTRA-radio coverage and E-UTRA-radio coverage. The figure shows a NodeB 14 of a UTRAN and an eNodeB 15 of an E-UTRAN. The NodeB 14 is connected to a network node of the UTRAN, i.e. an RNC 16, and the eNodeB is connected to a core network node 18. The UE 11 is located within an overlapping area of the UTRAN cell 12 and the E-UTRAN cell 13, wherein the UE may transfer from the E-UTRAN to the UTRAN, when it requests a CS service.
The transfer of the UE to the UTRAN due to the UE requesting a CS service could be performed e.g. according to a standardized CSFB (Circuit Switched Fall Back)-mechanism, which uses a PS HO (Packet Switched Hand Over) or a RRC Release operation. However, no specific mechanism is standardized for transferring the UE back to E-UTRAN after a CSFB, when the CS service has ended. Instead, when the CS service is terminated, and no PS (Packet Switched) session is ongoing, the UE is expected to go to Idle mode, and return to the E-UTRAN using e.g. a standardized LTE Cell Reselection. The Cell Reselection can also take place if the UE is in a CELL_PCH-state or in a URA_PCH-state, but not in a CELL_FACH-state.
Besides from the above-described situation, in which an LTE capable UE is connected to an UTRAN because a requested CS service is not available in E-UTRAN, other situations may exist in which an LTE capable UE may be connected to a UTRAN instead of to an E-UTRAN, such as e.g. when the UE does not have any LTE coverage, or due to operator strategies or load issues in the LTE network. However, when the reason for the connection to the UTRAN is not valid anymore, the UE should normally return to E-UTRAN as soon as possible.
Various methods exist for transferring a UE between two radio access network with different radio access technologies, such as e.g. from UTRAN to E-UTRAN, and different methods may apply for different UE states. The conventional method for a UE to transfer from a non-preferred RAT to the preferred RAT, in case the UE is in Idle mode or is connected on a common channel, is the above-mentioned LTE Cell Reselection, which requires that the UE is in the Idle mode, in a URA_PCH state or in a CELL_PCH state, i.e. that the UE has stopped transmitting data over the air, since the UE at this point is free to search for the most suitable available cell, with the highest prioritized radio access technology.
There is a need to improve handover between UTRAN and E-UTRAN.